1. Field of the Invention
The present invention relates to a metal casting fabrication method applicable for forming a metal housing of a notebook computer, a mobile telephone or the like. The present invention also relates to a metal casting produced by such a method.
2. Description of the Related Art
Mobile devices such as notebook computers and cellular phones should not weight very much. For the purposes of reducing weight (and some other purposes as well), their housings may be made of lightweight metal such as magnesium alloy or aluminum alloy. Since great precision is possible, such a metal housing is often formed by die-casting, whereby molten metal is injected under pressure into a cavity (“die cavity”) defined by dies, or molds. A forming technique by die-casting is disclosed in JP-A-9(1997)-272945 for example.
Though great precision is attained, die-casting has a drawback as follows. Specifically, molten metal injected into the die cavity will harden by being cooled by the cold dies. The problem occurs when the die cavity includes a narrow portion (whose width is smaller than 1.5 mm for example). Since the narrow portion cools the molten metal quickly, the metal impelled into the narrow portion may harden prematurely before it fills the narrow portion. Accordingly, an unfilled space is left in the die cavity.
The above problem may be addressed by a method disclosed in JP-A-2000-223855. In accordance with the teaching of JP-A-2000-223855, a metal object including small-width portions is formed by the combination of a die-casting and a non-die-casting techniques. Specifically, a metal object to be produced may include a first narrow portion and a second narrow portion continuous with the first narrow portion. The second narrow portion has a smaller width than the first narrow portion. To produce this metal object, the second narrow portion is prepared beforehand, separately from the first narrow portion, by a non-die-casting technique. The obtained second narrow portion is placed in the die cavity. Then, molten metal is injected into the die cavity. As a result, the broader first narrow portion will be formed in contact with the inserted second narrow portion.
In the method of JP-A-2000-223855, however, the first narrow portion is still formed by die-casting. Therefore, the above-mentioned problem (the occurrence of an unfilled space) may result in the first narrow portion. Another problem is caused by the direct contact of the second narrow portion with the dies. In this contact arrangement, the heat of the molten metal dissipates easily via the second narrow portion. As a result, the mechanical properties of the first narrow portion fail to be uniform in a region thereof adjacent to the joint between the first and the second narrow portions. Disadvantageously, this makes unstable the connection of the first narrow portion to the second narrow portion.
JP-A-5(1993)-177333, JP-A-7(1995)-255607 and JP-A-11(1999)-104798 also teach methods whereby a metal member is inserted in the die cavity before injection of molten metal is performed. These techniques, however, have been proposed in view of improving the surface condition of magnesium alloy or aluminum alloy, which has poor heat and corrosion resistance, but not for the purposes of forming a thin-walled portion properly by die-casting.